The present invention is a descendent of exercise machines that utilize body gravity as a resistance force. Typically, they comprise a frame to support an inclined rail or rails which operate to track a body sled or carriage from a point of origin to a point of elevation. The sled, in turn, supports the torso, releasing the arms and/or legs to apply bodily force through conventional leverage systems to effect elevation of the body sled against the force of gravity. Examples of the above described machines are found in the patents of Coyle, U.S. Pat. No. 4,176,836, issued on Dec. 4, 1979, Van Straaten, U.S. Pat. No. 4,911,438, issued on Mar. 27, 1990, and Rasmussen, U.S. Pat. No. 5,334,120, issued on Aug. 2, 1994. These patents, and others, demonstrate continuous evolution in the utility of these machines, broadening their application with an increased emphasis on human engineering. Not withstanding such progress, experiments with a number of exercise clients, using several state of the art machines of this class, reveal a need for continuing biomechanical improvements including: (1) A need to selectively reduce gravitational resistance of a machine during that phase of the cycle when there is a transition in body mechanics that places the body in an awkward position, for example, such a transition occurs at the approximate midpoint of one exercise cycle when a pulling down motion, involving the biceps, is switched to a pushing down motion involving the triceps. (2) The need to develop a resistance control system, separate from body gravity, as an auxiliary method for incrementally increasing or decreasing the bodily effort required to elevate the sled, thereby accommodating individual strength parameters. (3) The need to increase the versatility of this class of machines with structure that allows alternate body cycles or positions to increase the participation of muscle groups not originally served with a single cycle. (4) The need to continue the development of low impact, traction oriented machines that reduce shock and compression on the spinal column and joints. The present invention addresses the cited needs.